Fine particles having diameter are formed in many industrial processes and combustion processes. For various reasons these fine particles are measured. The fine particle measurements may be conducted because of their potential health effects and also for monitoring operation of industrial processes and combustion processes, such as operation of combustion engines, especially diesel engines. Another reason for monitoring fine particles is the increasing use and production of nanosized particles in industrial processes. The above reasons there is need for reliable fine particle measurement equipments and methods.
One prior art method and apparatus for measuring fine particles is described in document WO2009109688 A1. In this prior art method clean, essentially particle free, gas is supplied into the apparatus and directed as a motive fluid flow via an inlet chamber to an ejector provided inside the apparatus. The clean gas is further ionized before and during supplying it into the inlet chamber. The ionized clean gas may be preferably fed to the ejector at a sonic or close to sonic speed. The ionizing of the clean gas may be carried out for example using a corona charger. The inlet chamber is further provided with a sample inlet arranged in fluid communication with a channel or a space comprising aerosol having fine particles. The motive fluid flow (i.e. the clean gas flow) causes suction to the sample inlet such that a sample aerosol flow is formed from the duct or the space to the inlet chamber. The sample aerosol flow is thus provided as a side flow to the ejector. The ionized clean gas charges at least a fraction of the particles. The charged particles may be further conducted back to the duct or space containing the aerosol. The fine particles of the aerosol sample are thus monitored by monitoring the electrical charge carried by the electrically charged particles. Free ions are further removed using an ion trap arranged downstream of the ejector. A typical ejector (or ejector pump) includes a diverging cone (or diverging outlet diffuser) after the narrow throat to convert the kinetic energy of the gas to pressure. This increases the the size and residence time of the gas inside the ejector. The residence-time increase slowers the time response of the measurement apparatus based on the prior-art method (WO2009109688 A1).
One important demand for the fine particle monitoring apparatuses is reliable operation such that they may be operated long time periods without need for maintenance. In many applications, such as monitoring fine particles of combustion engines, it is also preferable that the monitoring apparatus may be operated continuously for conducting fine particle measurements in real-time. Furthermore, in many cases the fine particle monitoring apparatuses have to be installed in existing systems in which there is only limited amount of space for the particle measurement apparatus. Usually industrial systems, combustion systems or other aerosol comprising systems are designed as compact as possible. Therefore, the fine particle measurement apparatus also has to be small sized. The advantage of the small size is not, however, restricted only to the use of limited space. A more important advantage of the small size is the minimization of particle losses into the measurement apparatus. Further, small size enables faster time response of the measurement due to the faster gas flow through the small sensing volume.
In many cases it's important that manufacturing costs of the apparatus is low. For this reason the structure of the apparatus should not be too complex to fabricate.